Process for removing fine impurities from a fibrous suspension

ABSTRACT

Apparatus for removing fine impurities from a fibrous suspension comprises a dissolution device, a pre-purification device, a wet straining device, and at least one flotation device. The wet straining device comprises at least two sorting devices present in sequence, each sorting device having a wire for separating the fibrous suspension into an “accepts” portion and a “rejects” portion. The flotation device is adapted to purify the “accepts” portion of at least one sorting device, and further separates the fibrous suspension into an “accepts” portion and a “rejects” portion. The process for removing fine impurities from a fibrous suspension comprises the steps of subjecting the fibrous suspension to a dissolution process, subjecting the fibrous suspension to a pre-purification process, purifying the fibrous suspension in a wet straining process, and purifying “accepts” portions from the wet straining process by a flotation process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 of GermanPatent Application No. 198 02 494.0, filed on Jan. 23, 1998, and GermanPatent Application No. 198 06 732.1, filed on Feb. 18, 1998, thedisclosures of which are expressly incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus and process for removingimpurities, and more particularly, to an apparatus and process forremoving fine impurities from a fibrous suspension.

2. Discussion of Background Information

Fibrous suspensions, particularly those obtained from recycled paperwhich are intended for paper production, contain undesirable attendantmaterials, also referred to as “interfering materials,” which must beremoved in the preparation system. The removal of these interferingmaterials is generally performed in various steps since a fibroussuspension purification in a single process step is rarely successful.

Wet straining processes have turned out to be particularly effective forthe removal of fine impurities. In such processes, the suspension isconveyed into a straining device and is divided into at least twoportions, an “accepts” portion, i.e., the materials that have passed thewire, and a “rejects” portion, i.e., the materials rejected due to theirsize. Generally, in the parlance of paper makers, such wet strainingprocesses are known as “sorting.” It is also known to subject therejected materials, i.e., the rejects portion, to one or more additionalsubsequent straining steps. This process is known as “multistepsorting.” Multistep sorting can be carried out at various points of thepaper stock preparation. Generally, the later the sorting occurs duringthe process, the finer the wire openings that can be used in theprocess.

Examples of multistep wet straining are demonstrated in the scientificpaper “Sortierung von Altpapierstoff zur Herstellung von graphischenPapieren” [Sorting of Recycled Paper Stock for the Manufacturing ofGraphic Papers] by R. Rienecker from the Wochenblatt firPapierfabrikation [Weekly for Paper Making] No. 23/24, 1997, pp. 1149 to1159.

Although a large number of different systems are already known and usedfor purifying paper stock suspensions, these systems are oftenunsuccessful in removing all interfering impurities from the suspension.In particular, very fine, adhesive impurities, which often cause greatdifficulties in the production of paper, even in small quantities, canonly be removed at great expense, if at all.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and process for removingfine impurities from a fibrous suspension. The apparatus comprises adissolution device, a pre-purification device, a wet straining device,and a flotation device. The wet straining device comprises at least twosorting devices installed in sequence, each sorting device having a wirefor separating the fibrous suspension into an “accepts” portion and a“rejects” portion. The flotation device is adapted to purify the“accepts” portion of at least one sorting device, and further separatesthe fibrous suspension into an “accepts” portion and a “rejects”portion.

The flotation device may be further adapted to purify the “accepts”portion of the last of the at least two sorting devices. Additionally,the flotation device may be adapted to receive the “accepts′ portions ofthe last two of the at least two sorting devices.

The apparatus may also comprise a finishing device adapted to receive atleast one “accepts” portion of the wet straining device. Furthermore,the apparatus may be adapted to receive at least one “accepts” portionof the flotation device.

The pre-purification device may be adapted to remove ink particles, andmay remove these ink particles by flotation Additionally, the sortingdevices of the wet straining device may be embodied as closed pressuresorters. The pressure sorters may each be equipped with wires havingfree wire openings, the free wire openings having slits with a width ofbetween about 0.08 and 0.5 mm. Alternatively, the free wire openings maybe holes with a diameter of between about 0.8 and 3 mm.

The flotation device may additionally comprise a selectively functioningflotation cell.

The process for removing fine impurities from a fibrous suspensioncomprises the steps of subjecting the fibrous suspension to adissolution process, subjecting the fibrous suspension to apre-purification process, and purifying the fibrous suspension in a wetstraining process. At the wet straining process, at least two sortingdevices are installed in sequence, each sorting device having a wire forseparating the fibrous suspension into an “accepts” portion and a“rejects” portion. The wet straining process comprises introducing a“rejects” portion rejected at the wire of a preceding sorting deviceinto the subsequent sorting device until a last subsequent sortingdevice has been reached, conveying the “rejects” portion out of theprocess, and conveying the “accepts” portion to a flotation process. The“accepts” portion is purified by at least one flotation process, whichfurther separates the fibrous suspension into an “accepts” portion and a“rejects” portion, the “rejects” portion containing a majority of theimpurities of the fibrous suspension, and the “accepts” portioncontaining a majority of the fibers of the fibrous suspension.

The “accepts” portions of the sorting devices may contain the highestpercentage of interfering materials that can be floated and may bepurified in the flotation process.

Additionally, the “accepts” portion of the last subsequent sortingdevice may be purified in the flotation process. Alternatively, the“accepts” portions of the last two steps of the wet straining processmay be supplied to the flotation step process.

The “accepts” portions of the wet straining process that are notpurified in the flotation process may also be combined and supplieddirectly to a finishing process. Also, the “accepts” portion of theflotation process may be supplied to the remaining “accepts” portions ofthe wet straining process.

The pre-purification process may remove ink particles, and may removethese ink particles by flotation

The sorting devices of the wet straining process may be closed pressuresorters. These pressure sorters may be equipped with wires having freewire openings having slits with a width of between about 0.08 and 0.5mm. Alternatively, the free wire openings may be holes with a diameterof between about 0.8 and 3 mm.

At the beginning of the wet straining process, the dry matter content ofthe fibrous suspension may be set to a value of between about 0.6 and2.5%, or alternatively, may be set to a value of between about 2 and 4%.

The flotation step may be performed in a selectively functioningflotation cell.

The suspension to be purified may further contain adhesive particles.

Before the he flotation step, the suspension may be subjected to achemical treatment in which surface activity of the interferingmaterials is increased.

The fibrous suspension may be dispersed before the flotation process oralternatively, before the wet straining process. A specific work of atleast 20 kWh/to may be transmitted to the fibrous suspension during thedispersion of the fibrous suspension.

Other exemplary embodiments and advantages of the present invention maybe ascertained by reviewing the present disclosure and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further described in the detailed descriptionwhich follows, in reference to the noted plurality of drawings by way ofnon-limiting examples of certain embodiments of the present invention,in which like numerals represent like elements throughout the severalviews of the drawings, and wherein:

FIG. 1 is a basic process schematic for carrying out the processaccording to an embodiment of the present invention;

FIG. 2 is a basic process schematic for carrying out the processaccording to a second embodiment of the present invention;

FIG. 3 is a basic process schematic for carrying out the processaccording to a third embodiment of the present invention;

FIG. 4 is a basic process schematic for carrying out the processaccording to a fourth embodiment of the present invention;

FIG. 5 is a basic process schematic for carrying out the processaccording to a fifth embodiment of the present invention;

FIG. 6 is a basic process schematic for carrying out the processaccording to a sixth embodiment of the present invention; and

FIG. 7 is a basic process schematic for carrying out the processaccording to a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the embodiments of the present invention onlyand are presented in the cause of providing what is believed to be themost useful and readily understood description of the principles andconceptual aspects of the present invention. In this regard, no attemptis made to show structural details of the present invention in moredetail than is necessary for the fundamental understanding of thepresent invention, the description taken with the drawings makingapparent to those skilled in the art how the several forms of thepresent invention may be embodied in practice.

In multistep wet straining, rejects of a sorting step are introducedinto a subsequent sorting step. The accepted stock quality differsdepending on whether the step is the first, second, or a subsequentstep. The content of the interfering materials of the accepted stockgenerally increases from step to step. A certain amount of interferingmaterials to be sorted will invariably get into the accepted stock, evenif this amount is small. The accepted stock of the last step has thehighest comparative percentage of interfering materials because of theincrease in the dirt content from step to step, especially withinterfering materials that are very fine or are not very dimensionallystable. These interfering materials can easily get into the acceptedstock during the wet straining process. In particular instances, thecontent of the interfering materials can also be the highest in theaccepted stock of the sorting device that does not belong to the laststep.

The “accepts” quantity decreases from step to step and the percentage ofinterfering materials increases. Fine interfering materials can beseparated out effectively by a flotation process provided that thematerials are hydrophobic. These materials include adhesives (alsocalled “stickies”). The present invention economically removes much ofthis fine interfering material.

The present invention uses a flotation process, which functionsselectively, i.e., it leaves the hydrophilic fibers in the acceptedstock and concentrates on only the interfering materials in theflotation foam. Selective flotation processes of this kind are widelyknown in the art and are, therefore, not explained in detail. Thesuspension is prepared before the actual flotation so that as great aspossible a percentage of the interfering materials contained therein canbe floated. The preparation of the suspension can be executed eithermechanically or chemically. For example chemical means may be used toincrease the surface activity of the materials to be floated out, inorder to intensify their hydrophobic nature.

Referring to the drawings wherein like numerals represent like elements,FIG. 1 shows the division of the process into the individual steps:dissolution 1 (by a dissolution device), pre-purification 2 (by apre-purification device), wet straining 3 (by a wet straining device),flotation 4 (by a flotation device), and finishing 5 (by a finishingdevice). In dissolution 1, paper stock to be processed is mixed withwater and broken down to the point that a pumpable suspension isproduced. This suspension may also be speck-free. Pre-purification 2 isalso carried out early in the process, e.g., with the aid ofhydrocyclones or junk sorters. Removal of printing inks, known asde-inking, can also be carried out at the pre-purification step 2. Thedissolution 1 can also be continuously performed at the de-inkingprocess. The suspension can be deflaked or dispersed and thus preparedfor the wet straining process 3. During dispersion, a specific work ofat least 20 kWh/to is transmitted to the fibrous suspension. Thedissolution 1 and pre-purification 2 processes are usually not separateand discrete, as shown in the Figures, but take place in partial stepsand in an alternating manner.

After execution of these dissolution 1 and pre-purification 2 steps, thesuspension S travels into the multistep wet straining device 3. In theexample shown, the wet straining device 3 includes a respective firstsorting device 7, a second sorting device 7′ and a third sorting device7″. At least one wire is disposed in each of the sorting devices 7, 7′,7″, which divides the incoming suspension S into an “accepts” portion 9,9′, or 9″ and a “rejects” portion 8, 8′, or 8″. Each sorting device 7,7′. 7″ performs a respective first, second and last sorting step 6. Thesorting devices 7, 7′, 7″ are connected so that the subsequent steprespectively receives the “rejects” portion of the preceding step. Inthe present invention, the “rejects” portions 8, 8′, or 8″ may vary inmagnitude from each other.

The wet straining process 3 shown in FIG. 1 has three steps. The“accepts” portion 9″ of the last step 6 is treated in by a flotationdevice 4 in a flotation step, for which a flotation cell 12 is used. Thefine interfering materials are removed as completely as possible in the“rejects” portion 1, and the purified fibrous suspension travels as an“accepts” portion 10. together with the remaining “accepts” portions 9,9′, into the finishing device 5 of the finishing step.

While FIG. 1 shows a three step wet straining 3, it is readilyappreciable by those skilled in the art that more or fewer than threewet straining steps may be used in alternative embodiments. FIG. 2 showsa two-step wet straining process 3. The number of wet straining steps 3used in the invention depends on individual specific conditions andrequirements, and the needs of the user.

As shown in FIG. 3, the sorting device 7′ of the wet-straining step 3may also be combined with a flotation step 4. Thus, the wet strainingprocess 3 can be made more efficient when excessive interfering materialquantities are already contained by the sorting device 7′ of thewet-straining step 3. In certain instances, a last step 6 is performedat the sorting device 7″ (e.g., by means of very fine wire openings) sothat few interfering materials get into the “accepts” portion at thisstep. Additionally, the flotation of the “accepts” portion of thesorting device 7′ of the wet-straining step 3 alone may suffice.

The “accepts” portion 9′ of the sorting device 7′ (i.e., the secondstep), together with the “accepts” portion 9″ of the sorting device 7″(i.e., at the last step 6), could also be introduced into a singleflotation cell 12 in alternative embodiments.

In the embodiment shown in FIG. 4, the accepted stock of the sortingdevice 7′ (i.e., the second step) is improved. Two sorting devices 7′are connected in series with respect to the accepted stock and thecombined “rejects” 8″ are supplied into the sorting device 7″ of thelast step 6. The “accepts” portion 9′ of the second step is thencombined with the accepts” portion 9 from the first step.

FIG. 5 shows a variant of multistep wet straining 3. In this variant,the “rejects” 8″ from the last step 6 are introduced into an additionalsorting device 13. An “accepts” portion of this additional sortingdevice 13 is returned and travels into the sorting device 7″, togetherwith the “rejects” portion 8 of the first step, into the sorting device7″ of the last step 6. The “rejects” portion 14 of the sorting device 13is conveyed out of the system.

Referring to FIG. 6. the “accepts” portions 9″ of last two steps 6 and6′ are purified in the flotation step 4. Step 6′ is performcd at anintermediate sorting device 7′″. The embodiment of FIG. 6 is useful inthat oftentimes, there are impurities in the “accepts” portion of thesecond sorting device 7′. There is thus an additional opportunity forthe removal of impurities after the suspension leaves the second sortingdevice 7′. The sorting devices 7′ of the second step can be operated ata higher flow rate since the suspension S is further purified further asan “accepts” portion. As a result, the last sorting device 7″ is smalleror it can be operated more effectively.

FIG. 7 shows an additional embodiment of the pre-purification device 2.A majority of ink particles are removed from the fibrous suspension,e.g., by means of flotation. The process according to the inventionleads to an improvement in stock quality. The flotation step 4 isparticularly effective if it can be exclusively tailored to thoseinterfering materials that can be floated, such as adhesive andhot-melting interfering materials, known as “stickies,” which, unlikeinks, can often be easily removed and with fewer chemicals. Theflotation 4 step is especially useful if inks need not be entirelyremoved, e.g. raw materials for packaging papers.

The sorting devices 7, 7′, 7″ are influenced by stock density and thesize of the sorting device wire openings. Fibrous suspensions are oftensorted in a preparation system, e.g., with stock densities ofapproximately 2 to 4% and perforated wires of approximately 2 mm.Furthermore, suspensions are also sorted at other locations with slitsthat have a width of approximately 0.2 mm and stock densities underabout 1%. The process according to the invention can be used for bothabove examples, wherein results often differ. In sorters with slits ofapproximately 0.2 mm in width, stock densities of around 2 to 3% arestill possible. These densities lead to a relatively large massthroughput with a favorable sorting quality and, therefore, lead tooptimal and economical operation. However, as problems often arise withadhesive interfering materials (“stickies”), the process according tothe invention is particularly useful.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the present invention has been describedwith reference to certain embodiments, it is understood that the wordswhich have been used herein are words of description and illustration,rather than words of limitation. Changes may be made, within the purviewof the appended claims, as presently stated and as amended, withoutdeparting from the scope and spirit of the present invention in itsaspects. Although the present invention has been described herein withreference to particular means, materials and embodiments, the presentinvention is not intended to be limited to the particulars disclosedherein; rather, the present invention extends to all functionallyequivalent structures, methods and uses, such as are within the scope ofthe appended claims.

What is claimed is:
 1. A process for removing fine impurities from afibrous suspension, comprising: a) subjecting the fibrous suspension toa dissolution process; b) subjecting the fibrous suspension to apre-purification process; c) purifying the fibrous suspension by a wetstraining process, wherein at least two sorting devices are present insequence, each sorting device having a wire for separating the fibroussuspension into an “accepts” portion and a “rejects” portion, the wetstraining process comprising: introducing a “rejects” portion rejectedat the wire of a preceding sorting device into a subsequent sortingdevice until the “rejects” portion has been introduced into a lastsubsequent sorting device; conveying the “rejects” of the lastsubsequent sorting device portion out of the process; conveying the“accepts” of the last subsequent sorting device portion to a flotationprocess; d) purifying the “accepts” portion of the last subsequentsorting device by at least one flotation process, the flotation processfurther separating the fibrous suspension into an “accepts” portion ofthe flotation process and a “rejects” portion of the flotation process,the “rejects” portion of the flotation process containing a majority ofthe fine impurities of the fibrous suspension, and the “accepts” portionof the flotation process containing a majority of the fibers of thefibrous suspension; and e) combining and supplying, to a finishingprocess, the “accepts” portions of at least the preceding sorting devicewithout flotation and the “accepts” portion of the flotation process;wherein the dissolution and pre-purification processes are performedbefore the wet straining process.
 2. The process according to claim 1,comprising purifying, in the flotation process, the “accepts” portionsof the sorting devices that contain the highest percentage ofinterfering materials that can be floated.
 3. The process according toclaim 1, comprising supplying the “accepts” portions of the last twosubsequent sorting devices of the wet straining process to the flotationstep process.
 4. The process according to claim 1, comprising combiningand supplying, to a finishing process, each “accepts” portion of the wetstraining process that is not purified in the flotation process.
 5. Theprocess according to claim 4, comprising supplying the “accepts” portionof the flotation process to the remaining “accepts” portions of the wetstraining process.
 6. The process according to claim 1, comprisingremoving ink particles at the pre-purification process.
 7. The processaccording to claim 6, comprising removing, by flotation, said inkparticles at the pre purification process.
 8. The process according toclaim 1, comprising performing the flotation step without chemicals thatare particularly used to remove inks.
 9. The process according to claim1, comprising performing the flotation step without flotation chemicals.10. The process according to claim 1, wherein the sorting devices of thewet straining process are closed pressure sorters.
 11. The processaccording to claim 10, wherein the pressure sorters are equipped withwires having free wire openings, the free wire openings having slitswith a width of between about 0.08 and 0.5 mm.
 12. The process accordingto claim 10, wherein the pressure sorters are equipped with wires havingfree wire openings, the free wire openings being holes with a diameterof between about 0.8 and 3 mm.
 13. The process according to claim 10,comprising maintaining the dry matter content of the fibrous suspensionat the beginning of the wet straining process between about 0.6 and2.5%.
 14. The process according to claim 10, comprising maintaining thedry matter content of the fibrous suspension at the beginning of the wetstraining process between about 2 and 4%.
 15. The process according toclaim 1, wherein the flotation step is performed in a selectivelyfunctioning flotation cell.
 16. The process according to claim 1,wherein the original untreated fibrous suspension to be purifiedcontains adhesive particles.
 17. The process according to claim 1,comprising subjecting the suspension, before the flotation step, to achemical process in which surface activity of the interfering materialsis increased.
 18. The process according to claim 1, comprisingdispersing the fibrous suspension before flotation.
 19. The processaccording to claim 18, comprising transmitting a specific work of atleast 20 kWh/to to the fibrous suspension during the dispersion.
 20. Theprocess according to claim 1, comprising dispersing the fibroussuspension before the wet straining.